1. Field of the Invention
This invention is concerned with double layer capacitors and, more particularly, with an especially suitable electrolyte for the capacitor.
2. Description of the Prior Art
Double layer capacitors are disclosed in prior U.S. patents including:
U.S. Pat. No. 3,536,963 PA1 U.S. Pat. No. 3,652,902 PA1 U.S. Pat. No. 4,327,400 PA1 U.S. Pat. No. 4,394,713
In general, the above patents describe double layer capacitors which comprise a pair of polarization electrodes having a separating medium therebetween. The electrodes are composed of a solid and liquid phase and the electric double layer which characterizes these capacitors is formed at the interface between the solid and liquid (electrolyte) phases of the electrodes. The separating medium acts as an electronic insulator between the electrodes, but is sufficiently porous to permit ion migration therethrough.
Double layer capacitors can be made in miniature size, yet they exhibit very large capacitance when compared with conventional capacitors of similar or near similar size.
In the following description, reference will be made to the "leakage current" of capacitors. "Leakage current" is defined as the internal mechanism by which the capacitor self-discharges. It is measured by determining the current required to maintain the capacitor at a given charging voltage.
The prior art double layer capacitors having highly acidic electrolytes are found to work well for applications which require a current discharge above about 1 .mu.A. Where the particular application calls for a current discharge below this level, the internal leakage current of these prior art capacitors approaches, and may exceed, the required external current. So, for these low-current drain applications, the prior art capacitors will fail to deliver the required external current for useful periods of time.
An important consideration when judging the relative merits of capacitors is the time it takes for the voltage of charged capacitors to fall from some selected value to a selected lower value at a specified current discharge rate. The time consumed in discharging a capacitor to the extent specified is termed its "back up time". Back up times of capacitors are used as one facet of comparing performance.
A selected discharge current might be say, 1 .mu.A, in an electric circuit set up to compare the back-up times of capacitors. The back up time of a prior art double layer capacitor comprising a stack of six unit cells initially charged to 5 volts and discharged at 1 .mu.A is found to be less than 50 days; the final selected voltage being 2 volts.
Where charged capacitors are to be stored for varying periods prior to use, the leakage current becomes significant because this determines the "shelf life" of the unit. It would be especially valuable to have a capacitor which would reliably retain a useful voltage level for storage periods of several months or longer.